1. Field of Invention
The present invention relates generally to a method and apparatus for use in a semiconductor wafer processing system. More specifically, the inventive apparatus and method relates to a semiconductor substrate support having a guard electrode that reduces the amount of free electrons on the backside surface of the support.
2. Background of Prior Art
Brazing is a well known and commonly used method of fastening components in a semiconductor wafer processing system. Such brazing is useful in a semiconductor pedestal assembly for attaching metal components to a ceramic wafer support (i.e., an electrostatic chuck, a ceramic body, a heater and the like) in instances where a vacuum-tight seal is required to maintain the isolation of a process environment from a non-process environment. One such electrostatic chuck is disclosed in commonly assigned U.S. Pat. No. 5,511,799, issued Apr. 30, 1996, and is hereby incorporated by reference.
Patent ""799 discloses a sealing device useful in semiconductor processing and is disclosed as being particularly useful for creating two separate atmospheres or environments within a semiconductor processing chamber. The first environment (i.e., process environment) is typically maintained in a vacuum condition during wafer processing. The second environment (i.e., non-process or backside environment) is filled with a heat transfer medium. The heat transfer medium is typically an inert gas or vented atmospheric air. The presence of the heat transfer medium in the backside environment enhances and assists in heat exchange between a semiconductor wafer and a substrate support. Patent ""799 teaches a method which enables the ceramic components of the pedestal to be brazed to the metallic components of the pedestal while retaining an air-tight integrity over a wide range of temperatures.
One class of materials that is commonly used for brazing the above described electrostatic chuck components comprises silver and silver alloys. The silver alloy braze has proven generally to have good durability and reliability over time. However, in some instances while under harsh operating conditions within the semiconductor processing system, the silver, or silver component of the braze, has been found to separate and migrate from the brazed joint. If the brazed joint is in the proximity of other electrical connectors on the backside in the electrostatic chuck, the silver which has separated from the braze creates an undesired conductive path that shorts these electrical connectors or deteriorates the performance of the effected electrical circuits.
More specifically, the silver component of the braze alloy forms an oxide when exposed to environments containing oxygen. At chuck operating temperatures (i.e., above 300xc2x0 C.), the silver oxide becomes unstable, breaks down and releases silver ions. The silver ions are relatively mobile in the electric field that exists proximate the electrical connectors. As such, the silver ions tend to move along the electric field lines between the connectors, growing outwards from a positively biased component such as a terminal. Under these conditions, the silver ions readily adopt free electrons creating a silver precipitate. The precipitated silver accumulates about the positive terminals and migrates towards objects having an opposite polarity.
As the ceramic substrates that generally comprise an electrostatic chuck exhibit a drop in bulk resistivity (increase in conductivity) when heated, electrons are liberated within the ceramic material and eventually move towards the backside of the electrostatic chuck. These electrons passing through the ceramic chuck bond with the silver ions. After long periods of time and repeated exposures of the brazed joint to high temperatures in the environment containing oxygen, the amount of silver becomes significant. The significant amounts of precipitated silver forms dendrites. These dendrites generally form at terminals on a circuit having the highest potential, and fan out across the backside of the electrostatic chuck towards ground (and other lower potential objects) creating a substantially conductive path. Substantially conductive meaning that although the path is comprised of discreet particles, or droplets, of silver, the proximity of the droplets to each other coupled with the voltage potential allows for charge transfer (arcing) and current flow. When enough precipitated silver becomes aligned between these terminals, the conductive path reduces the gap between the terminals to the point that arcing occurs. In extreme circumstances, the shorting of the electrodes residing within the chuck occurs.
Therefore, there is a need for a method and apparatus for reducing the migration of conductive material.
One aspect of the present invention generally provides a substrate support. In one embodiment, a substrate support includes a chuck body having a support side and a backside. A guard electrode is disposed within the chuck body proximate the backside of the chuck body. Another embodiment of the invention provides a substrate support including a chuck body having a support side and a backside. A guard electrode is disposed within the chuck body for attracting electrons proximate the backside of the chuck body.
In another aspect of the invention, a method for reducing the migration of conductive material on a substrate support is provided. In one embodiment, a method for reducing the migration of conductive material on a substrate support comprises disposing a guard electrode proximate a backside of the substrate support and applying a voltage to the guard electrode.